1. Field of the Invention
The present invention relates to a piezoelectric sensor employed for an acceleration sensor and the like.
2. Related Background Art
This kind of piezoelectric sensor is used for detecting acceleration or shock in hard disk drives, airbag systems, electronically controlled suspension systems, and the like, for example. It is very important for such a piezoelectric sensor to have a high-degree of reliability, a small number of parts, and a compact size.
Known as examples of documents disclosing techniques satisfying such a requirement include Japanese Patent Application Laid-Open Nos. HEI 8-29447, HEI 9-54111, and HEI9-264901. In the techniques disclosed in these known documents, a base member is provided with a depression and a device attachment, and a piezoelectric device is secured to the device attachment. The piezoelectric device is attached to the device attachment so as to form a gap between the side and bottom faces of the depression.
The techniques disclosed in the above-mentioned known documents employ a bimorph piezoelectric device as the piezoelectric device. The bimorph piezoelectric device is constituted, for example, by two single-plate piezoelectric devices stacked and combined together. It is difficult for this kind of bimorph piezoelectric device to increase its electric charge sensitivity, since its electrostatic capacity is relatively small.
For securing a large electrostatic capacity in order to increase the electric charge sensitivity, the bimorph piezoelectric device must be made larger, whereby the piezoelectric sensor having incorporated the bimorph piezoelectric device therein becomes greater in size.
It is an object of the present invention to provide a piezoelectric sensor which can improve its electric charge sensitivity and reduce its size.
For achieving the above-mentioned object, the present invention provides a piezoelectric sensor comprising a piezoelectric device including a plurality of electrode films and a plurality of piezoelectric ceramic layers; a first support member having a first projection; and a second support member having a second projection; wherein the piezoelectric ceramic layers have a number of at least 4 and are alternately laminated with the electrode films; wherein the first and second support members are arranged such that the first and second projections are opposed to each other; and wherein the piezoelectric device is disposed between the first and second projections and held between an end face of the first projection and an end face of the second projection.
In the piezoelectric sensor in accordance with the present invention, as mentioned above, the first and second support members are arranged such that the first and second projections are opposed to each other. The piezoelectric device is disposed between the first and second projections, and is held by the end faces of the first and second projections. As a consequence, when an exciting force, a shock, or the like is applied, a portion of the piezoelectric device near its part held by the first and second projections deforms in response to the exciting force, thereby generating a detection signal.
Further, in the present invention, at least four piezoelectric ceramic layers are alternately laminated with the electrode films. Using these four or more piezoelectric ceramic layers can secure a larger electrostatic capacity, thus allowing the electric charge sensitivity to improve.
Also, the use of at least four piezoelectric ceramic layers can secure a larger electrostatic capacity without increasing the size of the piezoelectric device. As a consequence, the piezoelectric sensor incorporating the piezoelectric device therein can be prevented from increasing its size, whereby the piezoelectric sensor can be made smaller.
In the piezoelectric device, some of the electrode films may be connected to each other by way of a through hole formed in the piezoelectric ceramic layers. The use of the through hole can easily realize complicated interlayer connections between electrode films.
In a specific mode, the piezoelectric device has a longitudinally intermediate part held between the end faces of the first and second projections. When an exciting force, a shock, or the like is applied in this mode, parts on both sides of the intermediate part held by the first and second projections deform in response to the exciting force.
In another specific mode, one longitudinal end part of the piezoelectric device is held by the end faces of the first and second projections. When an exciting force, a shock, or the like is applied in this mode, a part on the end side opposite from the end part held by the first and second projections deforms in response to the exciting force.
In a preferred configuration of the piezoelectric sensor in accordance with the present invention, the first support member comprises a first contact electrode disposed at the end face of the first projection, whereas the second support member comprises a second contact electrode disposed at the end face of the second projection. In the piezoelectric device, the electrode films are disposed between the piezoelectric ceramic layers, and on both outer faces of the laminate formed by the piezoelectric ceramic layers, whereas the electrode films on both outer faces are electrically connected to the first and second contact electrodes by the holding. In such a configuration, detection signals generated upon application of exciting forces, shocks, and the like are taken from the electrode films on both outer faces to the outside by way of the first and second contact electrodes.
Other objects, configurations, and advantages of the present invention will be explained in further detail with reference to the accompanying drawings. However, these drawings are given by way of illustration only.